Cell Networking
16
1 Cell Networking Carey Williamson Department of Computer Science University of Calgary
description
Cell Networking. Carey Williamson. Department of Computer Science University of Calgary. Definition. The term “cell networking” means packet switching, but with fixed size packets (called cells) Contrast with “frame relay” which uses variable size packets - PowerPoint PPT Presentation
Transcript of Cell Networking
1
Cell Networking
Carey Williamson
Department of Computer ScienceUniversity of Calgary
2
Definition
The term “cell networking” means packet switching, but with fixed size packets (called cells)
Contrast with “frame relay” which uses variable size packets
ATM is an example of cell networking with 53-byte cells
3
Rationale for Cell Networks
Simpler interfaces Simpler buffering Simpler switches
» slotted system» synchronous stages
Better control of delay Better control of delay jitter
4
Delay Example
5
Delay Example (Cont’d)
Job 1, Size 10,arrives at time 0
Job 2, Size 2,arrives at time 3
+
=
Job 1 departs at time 10Job 2 departs at time 12
Server
Depart
6
Delay Example (Cont’d)
In the Internet, for example, a small packet (e.g., a telnet packet) that happens to get stuck behind a large packet (e.g., an ftp packet) can experience a large delay
FIFO service, non-preemptive
7
Delay Example (Cont’d)
Job 1, Size 10,arrives at time 0
Job 2, Size 2,arrives at time 3
+
=
Job 1 departs at time 12Job 2 departs at time 7
Server
Depart
(assumes jobsare equal priority)
8
Delay Example (Cont’d)
Job 1, Size 10,arrives at time 0
Job 2, Size 2,arrives at time 3
+
=
Job 1 departs at time 12Job 2 departs at time 5
Server
Depart
(assumes job 2is higher priority)
9
Advantages of Cells
High priority or delay-sensitive traffic will likely spend less time “stuck behind” other traffic
The smaller the cell, the better Lower mean delay, and lower variation
of delay Easier to provide performance
guarantees to integrated traffic
10
Summary
In addition to the ease of implementation considerations, cell based networks offer a better framework for providing delay guarantees on integrated traffic flows (e.g., data, voice, video)
That is why ATM uses cells
11
Why 53 bytes?
The smaller the cell, the better (in terms of delay guarantees)
Need to design for traffic with the most stringent delay requirements
Considerations for voice traffic were an overriding concern
12
Why 53 bytes? (Cont’d)
The ATM cell size was chosen by the CCITT international standards committee (now called ITU)
Influenced by voice traffic requirements and existing telco equipment in place at the time (e.g., echo cancellation)
13
Why 53 bytes? (Cont’d)
European community wanted 32 bytes of data per ATM cell
American community wanted 64 Result: compromise!
» (32 + 64) / 2 = 48 » thus, 48 bytes of data per ATM cell
Both sides equally (un)happy
14
Why 53 bytes? (Cont’d)
European community wanted 4 bytes of header per ATM cell
American community wanted 6 Result: compromise!
» (4 + 6) / 2 = 5 » thus, 5 bytes of header per ATM cell
48 + 5 = 53 bytes per ATM cell
15
Why 53 bytes? (Cont’d)
Equally inefficient for all types of traffic (data, voice, video)» data networks want big packets» ATM overhead is 5/53 = 10% (too high!)» voice networks want small(er) packets» 48 bytes @ 64 kbps = 6 msec» video probably wants big(ger) packets
16
Summary
53 bytes is now the international standard for ATM cell size
“Only a standards committee could come up with a packet size that is a prime!’’ (Raj Jain, 1993)
Live with it; everyone else is!!
